Onium hydroxides, such as tetramethylammonium hydroxide, are used in processing of semiconductors and in processing of flat panel display devices, e.g., liquid crystal display (LCD) devices. Environmental and economic concerns drive the desire to recycle materials, including materials such as the onium hydroxides used in processing of semiconductor and flat panel devices. Materials such as onium hydroxides and onium salts are the subject of rather stringent environmental regulations. The production of new onium hydroxides and onium salts is more expensive than is the recycling of used onium hydroxides and onium salts. Use of materials such as onium hydroxides in semiconductor and liquid crystal display processing inevitably contaminates the onium hydroxides with process residues such as photoresist residues. While removal of most process residues and recovery of the onium hydroxides from waste mixtures from such processing has been known, it has proven quite difficult efficiently to remove process residues such as photoresist residues and similar materials from such waste mixtures without creating problems in the recycling process. In ion exchange processes for recycling onium ions, problems include channeling, early breakthrough and resulting loss of ion exchange capacity, as a result of the presence of the process residues. In order to meet this need and overcome these problems, it has been generally necessary to conduct such recovery operations at a level of efficiency that is lower than that theoretically possible based on the loading of onium hydroxides in the waste mixtures and the known ion exchange capacity of the ion exchange resins used for such recovery, since heretofore it has been impossible to attain removal of such contaminants from recycled onium hydroxide-containing waste mixtures at higher efficiencies. Thus, it has proven difficult to recycle materials such as the onium hydroxides used in processing of semiconductor and flat panel display devices for re-use in subsequent processing of semiconductor and flat panel display devices. Removal of residues such as photoresist residues from recycled onium hydroxide and salt solutions has been a long-standing problem, and a need remains for such methods, if materials such as onium hydroxides and salts are to be successfully and economically recycled for use in the most demanding applications, such as in semiconductor and flat panel display processing.